This invention relates to acoustic detectors for identifying aircraft in a marine environment, and more particularly, to a new and improved acoustic detector and method of operation, the detector being of the type employing a surface-buoyed acoustic air resonator for providing early detection of propeller-driven reconnaissance type aircraft by underwater marine vehicles.
In the field of intelligence gathering, various methods have been utilized in an attempt to identify propeller-driven aircraft whose primary mission is reconnaissance of the open seas for locating underwater marine vehicles. Because of the inherent danger posed by approaching reconnaissance aircraft to the underwater vehicles, clandestine detection is desirable. Thus, in the past, substantial effort has been employed to identify certain characteristics exhibited by propeller-driven aircraft. One of those characteristics is the Blade Passage Rate sound wave generated by the propeller blades of distant aircraft. This Blade Passage Rate sound wave is the dominate radiated noise mechanism and is generated by the propeller blades of the aircraft cutting through air and has a characteristic frequency which can be identified for a particular type aircraft. Such a characteristic frequency can be determined by multiplying the number of propeller blades times the propeller revolutions per minute (RPM) and then dividing this product by sixty (60) to arrive at the characteristic frequency in cycles/second (Hz). Generally, such characteristic frequencies range below 100 Hz.
An obvious method of clandestine detection is to identify these characteristic frequencies by an underwater acoustic means. However, the detection of aircraft by such underwater acoustic means is severely limited in range due to the physics of aircraft sound propagation in the ocean. Consequently, the poor propagation of propeller characteristic sound frequencies beneath the surface of the ocean prevents aircraft detection with underwater vehicle sonar at sufficient ranges to permit evasive action by the vehicle. Generally, when sound waves strike the surface of the water obliquely, they are reflected. Sound waves can enter the ocean if the sound wave strikes the water on a vertical path which is nearly perpendicular to the surface of the water. If the sound wave does enter the ocean, it bounces in a surface and bottom bounce path in a zig-zag fashion resulting in substantial attenuation of the sound wave as it propagates horizontally. The aforementioned attenuation causes the sound waves to be indistinguishable from background noise to an underwater sensor located only a few miles away.
Much effort has been expended it an attempt to solve the above-described problem. For example, U.S. Pat. No. 4,533,945 issued to Lauvray et al. discloses an apparatus equipped with a television camera or with a radio or radar antenna and connected to an underwater vehicle by a cable. The apparatus or missile is initially ejected from a tube on-board the vehicle. The missile, which is highly buoyant and having a low hydrodynamic drag, and the cable rise at a high speed in a rotating fashion to the surface of the ocean and "springs" vertically out of the water to permit an on-board camera to scan the horizon.
U.S. Pat. No. 3,258,595 issued to Galante discloses a similarly sophisticated vertically oriented buoy that is operatively associated by laser communication with a submerged vehicle. The dome cover of the buoy extends vertically above the water line for permitting a television camera to detect surface and airborne craft.
U.S. Pat. No. 3,651,286 issued to Gorike et al. discloses a microphone assembly of the Lavalier type mounted within a casing in a manner to achieve the dampening of the noise which is transmitted through the casing. The improvement in the microphone assembly comprises a support for the forward end of the microphone capsule including a cap portion defining an outwardly extending annular collar adjacent to an acoustic inlet. Further, a resilient vibration-absorbing annular molding embraces the collar with the outer periphery of the annular molding engaging the inner surface of the casing. It is noted in Gorike et al. that the collar together with an air chamber defined between the cap and the diaphragm form a Helmholtz resonator designed to provide a damping effect at higher frequencies.
U.S Pat. Nos. 4,189,786 and 4,203,109, respectively issued to Adler and to Ballard et al., each disclose systems for communicating information between a submerged vessel and a buoy. The buoy, in turn, is capable of communicating with an aircraft. Alternately, U.S. Pat. No. 3,999,183 to Brett discloses a floatable radio antenna releasable from an underwater marine vehicle and which includes a solid hemisphere of conductive material concentric with a hollow hemisphere of dielectric material. The hemispheres are contained in a floatation jacket which carries them above the surface of a liquid on which the jacket floats. Concentric spheres can be used if desired. The dielectric material permits realization of an antenna of small physical dimensions.
No reference found in the prior art teaches a technique for identifying the characteristic frequencies of propeller-driven aircraft by an acoustic means. More importantly, the prior art does not teach long range detection of propeller-driven aircraft by acoustic means which would provide sufficient time for underwater marine vehicles to take evasive action. In particular, most of the methods do not appear to be clandestine and would tend to draw attention in crowded marine environment. Further, those techniques which include the transmission and reception of RF and UHF frequencies via a communication means would tend to disclose the position of the underwater marine vehicle.
Hence, those concerned with the development and use of acoustic detectors in the intelligence gathering field have long recognized the need for an improved acoustic detector for use in a marine environment which is capable of exploiting the sound amplification properties of the detector, is passive, clandestine, sensitive and simple in construction, is inexpensive to manufacture and deploy, overcomes the problems associated with underwater detection devices, and identifies the characteristic frequencies of propeller-driven aircraft at ranges which provide sufficient time for evasive action by the underwater marine vehicle. The present invention fulfills all of these needs.